The present invention relates to a method of maintaining the pressure difference in a heat treatment plant for liquid food products, of the type in which, in a production disruption, the insufficiently treated product is caused to recycle by a return stroke.
The method is intended to be used in a plant for heat treatment of the type which is described in Swedish Patent Specification SE 9203213-5. Such a heat treatment plant is intended for pasteurising liquid food products, such as milk. The plant essentially comprises a plate heat exchanger, divided into a cooler section, one or more regenerative sections and a heater section. The plant also includes a retainer cell, normally placed outside the heat exchanger, pumps, balance tank and a number of pressure and temperature meters. Further components which may also belong to the plant are a separator, standardisation equipment and a homogeniser. The various part included in the plant are interconnected to form a unit by means of conduits. The unit is a plant for heat treatment of liquid food products, a pasteuriser.
The heat treatment plant is intended to heat the product to a certain, preselected temperature and keep the product at this temperature for a given time. The temperature in pasteurisation is normally 72-75xc2x0 C. and the time interval is 15-20 seconds.
In the heat treatment plant which is intended, use is made of regenerative sections, i.e. use is made of the heated, ready-pasteurised product which is on its way out of the plant to heat up the cold product which is entering the plant. As a result of this procedure, a high thermal degree of efficiency will be attained in the plant. At the same time, care must be taken to ensure that the pasteurised product is at a higher pressure than the unpasturised product so that, in the event of untightness between the plates in the heat exchanger, there is no risk that unpasteurised product leak out into the pasteurised product. The problem may be wholly obviated by simply using water as the thermal transfer mediums, which, however, radically reduces the degree of efficiency of the plant. Use may also be made f a heat exchanger with double plates, having a space for drainage of leaking product but this renders the plant considerably more expensive.
Thus, the striving is to maintain an excess pressure on the pasteused product, which may constitute a problem since, in the event of production disruption, there may be a temporary pressure drop. The authorities in different countries will probably in future issue stricter regulations relating to pasteurisation safety and, as a result, the demands of being able to maintain the pressure difference so as not to run the risk that unpasteurised product infects the already pasteurinsed product.
In production disruptions such as varying temperature of the incoming product into the pasteuriser, brief disruptions in steam supply or variations in the fat content of the product, a product which has not reached the correct temperature may leave the heater section. Such incompletely heated product is recycled to the balance tank, or is alternatively recycled over a larger or smaller part of the plate heat exchanger. During this time, operational security must be sustained by a maintained excess pressure on the already pasteurised product. The longer the operational downtime, the more difficult it will be to maintain the excess pressure.
One object of the present invention is, in a simple and reliable manner, to ensure that the excess pressure is maintained on the pasteurised product when a temporary pressure drop occurs in connection with a production disruptions.
SOLUTION
This and other objects have been attained according to the present invention in that the method of the type described by way of introduction has been given the characterizing feature that the pressure difference after a return stroke is maintained in that the treated product is enclosed in a conduit on the downstream side of the plant, and that the space for the product there is restricted.
Preferred embodiments of the method according to the present invention have further been given the characterizing features as set forth in the appended subclaims.